Anodizing is a method of providing an anodic oxide coating on a metal substrate, often used in industry to provide a protective and sometimes cosmetically appealing coating to metal parts. During an anodizing process, a portion of the metal substrate is converted to a metal oxide, thereby forming a protective oxide layer or coating. The nature of the anodic oxide coatings can depend on a number of factors, including chemical makeup of the metal substrates and the process parameters used in the anodizing processes. Sulfuric acid based anodizing processes, such as “Type II” and “Type III” anodizing as defined by military MIL-A-8625 specifications, are widely used to provide porous anodic oxide coatings which are then sealed to give good wear and corrosion resistance on the aluminum substrates.
Although sealed anodic oxide coatings can provide good wear and corrosion resistance, they can be susceptible to cracking when exposed to high temperatures. This is due to the difference in thermal expansion coefficients between the oxide material and underlying metal substrate. In particular, the metal substrate generally has a much higher thermal expansion coefficient than an overlying anodic oxide. When an anodized part is exposed to higher temperatures, the metal substrate expands more than the overlying anodic oxide, creating tensile stress within the anodic oxide coating that can cause the anodic oxide coating to develop small cracks once cooled back down to room temperature. These small cracks may not only be cosmetically unappealing, but may also impair the corrosion protection of the anodic oxide coating. In some cases, temperatures as low as 80 to 100 degrees Celsius can cause crazing of an anodic oxide coating. What are needed therefore are anodic oxide coatings that are more resistant to thermally induced cracking.